It is often desired to wrap the separator material around either the positive or negative plates of an automotive storage battery and to seal the separator material around the edges of those plates so that in the event of misalignment or build-up of active material at the bottom of the battery during use, shorting such as treeing, mossing, etc. will not develop. Accordingly, storage batteries, and particularly automotive storage batteries, wherein either the positive or negative plates are encased in separator envelopes may utilize correspondingly lower rest-ups without danger that accumulated material on the bottom of the battery case will cause shorting and premature failure of the battery.
Since the advent of the storage battery industry, virtually hundreds of different materials have been suggested for use as separator material. For many years, wood was the material of choice, after which various paper materials received widespread acceptance. More recently, microporous synthetic separator materials have all but displaced previous separator materials, due in large part to the superior characteristics exhibited by these new materials. One such synthetic separator material which has gained widespread acceptance is marketed under the tradename "Duramic" and basically comprises a thin sheet of material having a plurality of longitudinal ridges disposed on one side thereof which, for automotive storage batteries, are spaced approximately one-half inch apart. Although the precise composition of the separator material is not widely known, it is a polymeric, polyvinylchloride-like material containing substantial amounts of diatomaceous earths which are carefully manufactured to control preferred microporosities.
This common type of separator material is formed with sharp, substantially rectangular, longitudinal ridges on a first side, while on the opposite side of the separator surface, a much flatter, slightly undulating surface is presented, which structure tends to add to the overall longitudinal rigidity of the separator material.
Several different machines have been proposed for enveloping plates of automotive storage batteries, most of which machines focus on the idea of taking strips cut from a continuous length of separator material and shoving a plate into a slot or similar opening over which the separator material has been placed, so as to fold the separator material from the bottom of the plate up over the face of the plate with the edges of separator material extending beyond the side edges of the plate. The separator material, which is somewhat wider than the plate, and which edges overlap the sides of the plate, may be ultrasonically welded or may be transferred to a different station for other sealing, such as gluing or heat sealing.
Unfortunately, due to the particular nature of the diatomaceous earth and other materials used in making the separator material, variation in the quality of the separator material to be joined have created substantial difficulties in using ultrasonic sealing techniques. Alternatively, some problems have been encountered in attempting to use a direct contact heat sealing approach due to the fact that substantial pressures, in the order of 4,000 pounds per square inch of contact area must be applied to effect a good seal, while at the same time, the contact surfaces must not stick to the softened material upon withdrawal therefrom. In order to overcome there difficulties, it has been known to apply spray silicone coatings or Teflon coatings to the contact melting surfaces for the purpose of creating an anti-stick surface. Unfortunately, the extreme pressures exerted between the melting surfaces during the sealing process encourage foreign material, dust particles, lead shavings and other contaminants to become embedded in the various anti-stick coatings which may be applied to the sealing surfaces. Consequently, upon withdrawal of the softened separator material away from the contact melting surface, portions of the softened separator material tend to adhere to this foreign material and be pulled from the separator, resulting in a defective product. Accordingly, to counteract this tendency, Teflon surfaces, which tend to decompose under conditions of heat and pressure, need to be replaced or resurfaced frequently, while silicone coatings need to be renewed as often as every several hours in order to overcome the aforementioned problems.